This forum is to be used for all discussions pertaining to The Elder Scrolls IV: Oblivion, its Knights of the Nine and Shivering Isles expansions, and any user-created or premium modules.
It's been a while since I played Oblivion so this might not be correct. However, if I am, you will gain entrance to the underground door (spoiler) by murdering someone. It's part of the Dark Brotherhood quest line.. It could be that's in Leyawin though...